1. Field of the Invention
The present invention relates to low pressure carburization and other heat treating processes applied to metal alloy parts and more particularly steel parts and to high temperature capable furnaces having the capability of providing in the same furnace chamber alternatively, low pressure (vacuum) and high pressure (gas quench) environments for such processes.
2. Background Art
Vacuum (low pressure) heat treating (carburization) of steel or high alloy-content steels has been accomplished over past decades using various heat treating processes. Some alloys are particularly difficult to treat and require post treatment, for example, quenching to finish the treatment. Some metals are more difficult to treat (for example alloys such as AISI grade 4140, 4340, 8620, and 9310). Work pieces containing such alloys are currently heat treated and then moved to an oil or salt bath quench. That is, the work pieces are moved, mechanically, from the hot zone at temperature, into an outer vestibule chamber and submerged into a tank filled with oil or salt to rapidly cool the work pieces. The pieces thus moved and quenched have problems with distortion. Also, cleaning the parts after they have been submerged in oil or salt is a costly challenge. The mechanism for moving the work pieces at temperature undesirably adds significant cost, time and maintenance issues to the process. Gas quenching has been used as a post treatment for carburization of steel parts. Although, gas quenching avoids much of the finish product cleanup issues, it does not avoid the mechanical movement of the workload from one chamber to another. It also is not without challenges in how it affects finished product quality. In regard to the carburization process early and ongoing processes involve using as the carburizing gas hydrocarbons, such as, a gaseous saturated aliphatic hydrocarbon, e.g. methane, propane and butane. The selection as to which hydrocarbon should be used as the carburizing agent has been an evolving debate. The selected gas would be added at a pressure, for example, of 10-700 torr in the carburizing chamber, and the parts “absorb” carbon on the surface. Next, the reactive gas is removed and the surface carbon is allowed to diffuse below the surface. With such hydrocarbon gases, however, soot produced in the carburizing chamber interferes with desired consistency of carburizing quality and adds significant cost to parts cleanup and furnace maintenance. Achieving a uniform carburized “case”, a hardened, uniform surface layer, has been difficult and costly. Uniformity has been a major challenge. Sandblasting parts prior to carburizing to get rid of surface oxidation prior to carburization became a routine requirement. Atmosphere carburization suffers from the added problem of surface oxidation during heat treatment. The use of moderately higher carburizing temperatures, compared to atmospheric carburizing conditions, over shorter carburizing times has, for example, been found to provide a more uniform oxide free carburized case depth, cleaner parts, less part distortion, and the elimination of post process machining. Over the years vacuum carburizing has become cost effective as compared to traditional atmosphere carburization. Conventional high temperature vacuum furnaces have been described in numerous prior art patents. Carburizing furnaces are in many respects similar to those conventional high temperature vacuum furnaces. In general, such furnaces are commonly of a substantially cylindrical shape having a substantially circular internal cross-section. Such a furnace is closed at its forward end by a releasable door, regularly with hinges so that the door swings out of the way for loading and unloading the furnace. The furnace doors have vacuum seals when closed to support the vacuum capability of the furnace. Also the doors regularly have insulation placed and formed to mate with insulation lining of the circular cross section furnace walls. Although the furnace of this invention has the above-mentioned features of prior art furnaces, and others, (See for example, U.S. Pat. No. 4,499,369, wherein a series of cylindrical resistance graphite heating elements are spaced longitudinally along the furnace interior and spaced from the walls.) key differences will be revealed in the following.
Consideration of the explosive and fire dangers associated with low molecular weight unsaturated hydrocarbons no doubt dissuaded some early carburization developers from attempting to use gasses such as acetylene and ethylene in carburizing applications. A relatively recent patent, U.S. Pat. No. 6,187,111 B1, (hereinafter the 111 patent) teaches away from the concept of using “acetylenic gas” as presenting “safety problems due to the combustibility of the gas.” That teaching is significant, in part because it apparently takes issue with earlier studies and patents much of which apparently does not deal with the dangers so conspicuous to the 111 patent authors. The 111 patent also teaches away from using hydrogen in carburizing applications, for example, as described in U.S. Pat. No. 5,205,873, also because of the safety issue. An early study, 1982 Jelle Kassperma and Robert H. Shay. (Metall. Trans. B 13B, 1982 267), presented an intensive study of the use of hydrocarbon gases as carburizing agents. The paper reveals investigation of the carburization reaction rates for methane, ethane, propane, ethylene and acetylene. The hydrocarbons were used in a conjunction with nitrogen as the carrier gas and hydrogen as an additive. The data supported acetylene as having the fastest rate for carburization and that propane is faster than ethylene. The investigators also provided an assessment of soot formation and the benefits of hydrogen in the mixture. An even earlier use of unsaturated hydrocarbons for carburizing, including acetylene, was disclosed in U.S. Pat. No. 3,988,955, issued Nov. 2, 1976: “Suitable carbonizing gases include methane, natural gas, propane, acetylene and benzene.” U.S. Pat. No. 4,035,203 also discloses the use of acetylene as an “active” gas for carburizing. About the same time Russian developers, recognizing problems associated with the use of aliphatic hydrocarbons in carburizing and the dangers of poor furnace construction, nonetheless looked to acetylene as the hydrocarbon of choice for carburizing. USSR Patent Specification No. 668978 (published patent specification date: Jun. 28, 1979, and referred to hereinafter as “USSR patent”) disclosed vacuum carburizing using acetylene at a pressure in the range of “0.01-0.95 atm.” (that is, 7.6 torr to 722 torr.). Interestingly, U.S. Pat. No. 5,702,540, (filed 15 plus years later, without referencing the USSR patent) claims using an acetylenic gas as the carburizing gas at a vacuum of not more than “1 kPa” (that is, not more than 7.5 torr). More recently, US Patent Application, US2003/0168125, disclosed a method for vacuum carburizing utilizing acetylene as the carburizing gas in the presence of a neutral carrier gas (N2 or H2) and requiring a pulsing sequence (i.e. boost/diffuse cycles). Reference is also made to the patent application filed on this date by William R. Jones et. al. entitled “Process For Heat Treating Steel Alloys” which is incorporated by reference in its entirety.